Both receptor tyrosine kinases and serine/threonine kinases have been implicated in cellular signaling pathways that control cell function, division, growth, differentiation, and death (apoptosis) through reversible phosphorylation of the hydroxyl groups of tyrosine or serine and threonine residues, respectively, in proteins. In signal transduction, for example, extracellular signals are transduced via membrane receptor activation, with amplification and propagation using a complex choreography of cascades of protein phosphorylation, and protein dephosphorylation events to avoid uncontrolled signaling. These signaling pathways are highly regulated, often by complex and intermeshed kinase pathways where each kinase may itself be regulated by one or more other kinases and protein phosphatases. The biological importance of these finely tuned systems is such that a variety of cell proliferative disorders have been linked to defects in one or more of the various cell signaling pathways mediated by tyrosine or serine/threonine kinases.
Receptor tyrosine kinases (RTKs) catalyze phosphorylation of certain tyrosyl amino acid residues in various proteins, including themselves, which govern cell growth, proliferation and differentiation.
Downstream of the several RTKs lie several signaling pathways, among them is the Ras-Raf-MEK-ERK kinase pathway. It is currently understood that activation of Ras GTPase proteins in response to growth factors, hormones, cytokines, etc. stimulates phosphorylation and activation of Raf kinases. These kinases then phosphorylate and activate the intracellular protein kinases MEK1 and MEK2, which in turn phosphorylate and activate other protein kinases, ERK1 and 2. This signaling pathway, also known as the mitogen-activated protein kinase (MAPK) pathway or cytoplasmic cascade, mediates cellular responses to growth signals. The ultimate function of this is to link receptor activity at the cell membrane with modification of cytoplasmic or nuclear targets that govern cell proliferation, differentiation, and survival. Mutations in various Ras GTPases and the B-Raf kinase have been identified that can lead to sustained and constitutive activation of the MAPK pathway, ultimately resulting in increased cell division and survival. As a consequence of this, these mutations have been strongly linked with the establishment, development, and progression of a wide range of human cancers. The biological role of the Raf kinases, and specifically that of B-Raf, in signal transduction is described in Davies, H., et al., Nature (2002) 9:1-6; Garnett, M. J. & Marais, R., Cancer Cell (2004) 6:313-319; Zebisch, A. & Troppmair, J., Cell. Mol. Life. Sci. (2006) 63:1314-1330; Midgley, R. S. & Kerr, D. J., Crit. Rev. Onc/Hematol. (2002) 44:109-120; Smith, R. A., et al., Curr. Top. Med. Chem. (2006) 6:1071-1089; and Downward, J., Nat. Rev. Cancer (2003) 3:11-22.
Naturally occurring mutations of the B-Raf kinase that activate MAPK pathway signaling have been found in a large percentage of human melanomas (Davies (2002) supra) and thyroid cancers (Cohen et al J. Nat. Cancer Inst. (2003) 95(8) 625-627 and Kimura et al Cancer Res. (2003) 63(7) 1454-1457), as well as at lower, but still significant, frequencies in the following:    Barret's adenocarcinoma (Garnett et al., Cancer Cell (2004) δ 313-319 and Sommerer et al Oncogene (2004) 23(2) 554-558),    billiary tract carcinomas (Zebisch et al., Cell. Mol. Life. Sci. (2006) 63 1314-1330),    breast cancer (Davies (2002) supra),    cervical cancer (Moreno-Bueno et al Clin. Cancer Res. (2006) 12(12) 3865-3866),    cholangiocarcinoma (Tannapfel et al Gut (2003) 52(5) 706-712),    central nervous system tumors including primary CNS tumors such as glioblastomas, astrocytomas and ependymomas (Knobbe et al Acta Neuropathol. (Berl.) (2004) 108(6) 467-470, Davies (2002) supra, and Garnett et al., Cancer Cell (2004) supra) and secondary CNS tumors (i.e., metastases to the central nervous system of tumors originating outside of the central nervous system),    colorectal cancer, including large intestinal colon carcinoma (Yuen et al Cancer Res. (2002) 62(22) 6451-6455, Davies (2002) supra and Zebisch et al., Cell. Mol. Life. Sci. (2006),    gastric cancer (Lee et al Oncogene (2003) 22(44) 6942-6945),    carcinoma of the head and neck including squamous cell carcinoma of the head and neck (Cohen et al J. Nat. Cancer Inst. (2003) 95(8) 625-627 and Weber et al Oncogene (2003) 22(30) 4757-4759),    hematologic cancers including leukemias (Garnett et al., Cancer Cell (2004) supra, particularly acute lymphoblastic leukemia (Garnett et al., Cancer Cell (2004) supra and Gustafsson et al Leukemia (2005) 19(2) 310-312), acute myelogenous leukemia (AML) (Lee et al Leukemia (2004) 18(1) 170-172, and Christiansen et al Leukemia (2005) 19(12) 2232-2240), myelodysplastic syndromes (Christiansen et al Leukemia (2005) supra) and chronic myelogenous leukemia (Mizuchi et al Biochem. Biophys. Res. Commun. (2005) 326(3) 645-651); Hodgkin's lymphoma (Figl et al Arch. Dermatol. (2007) 143(4) 495-499), non-Hodgkin's lymphoma (Lee et al Br. J. Cancer (2003) 89(10) 1958-1960), megakaryoblastic leukemia (Eychene et al Oncogene (1995) 10(6) 1159-1165) and multiple myeloma (Ng et al Br. J. Haematol. (2003) 123(4) 637-645),    hepatocellular carcinoma (Garnett et al., Cancer Cell (2004),    lung cancer (Brose et al Cancer Res. (2002) 62(23) 6997-7000, Cohen et al J. Nat. Cancer Inst. (2003) supra and Davies (2002) supra), including small cell lung cancer (Pardo et al EMBO J. (2006) 25(13) 3078-3088) and non-small cell lung cancer (Davies (2002) supra),    ovarian cancer (Russell & McCluggage J. Pathol. (2004) 203(2) 617-619 and Davies (2002) supr), endometrial cancer (Garnett et al., Cancer Cell (2004) supra, and Moreno-Bueno et al Clin. Cancer Res. (2006) supra),    pancreatic cancer (Ishimura et al Cancer Lett. (2003) 199(2) 169-173),    pituitary adenoma (De Martino et al J. Endocrinol. Invest. (2007) 30(1) RC1-3),    prostate cancer (Cho et al Int. J. Cancer (2006) 119(8) 1858-1862),    renal cancer (Nagy et al Int. J. Cancer (2003) 106(6) 980-981),    sarcoma (Davies (2002) supra), and    skin cancers (Rodriguez-Viciana et al Science (2006) 311(5765) 1287-1290 and Davies (2002) supra).
Overexpression of c-Raf has been linked to AML (Zebisch et al., Cancer Res. (2006) 66(7) 3401-3408, and Zebisch (Cell. Mol. Life. Sci. (2006)) and erythroleukemia (Zebisch et al., Cell. Mol. Life. Sci. (2006).
By virtue of the role played by the Raf family kinases in these cancers and exploratory studies with a range of preclinical and therapeutic agents, including one selectively targeted to inhibition of B-Raf kinase activity (King A. J., et al., (2006) Cancer Res. 66:11100-11105), it is generally accepted that inhibitors of one or more Raf family kinases will be useful for the treatment of such cancers or other condition associated with Raf kinase.
Mutation of B-Raf has also been implicated in other conditions, including cardio-facio cutaneous syndrome (Rodriguez-Viciana et al Science (2006) 311(5765) 1287-1290) and polycystic kidney disease (Nagao et al Kidney Int. (2003) 63(2) 427-437).